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KEY CONCEPT
Chemical reactions
involve energy changes.
BEFORE, you learned
NOW, you will learn
• Bonds are broken and made
during chemical reactions
• Mass is conserved in all
chemical reactions
• Chemical reactions are
represented by balanced
chemical equations
• About the energy in chemical
bonds between atoms
• Why some chemical reactions
release energy
• Why some chemical reactions
absorb energy
VOCABULARY
EXPLORE Energy Changes
bond energy p. 86
exothermic reaction p. 87
endothermic reaction p. 87
photosynthesis p. 90
How can you identify a transfer of energy?
PROCEDURE
1 Pour 50 ml of hot tap water into the cup
and place the thermometer in the cup.
2 Wait 30 seconds, then record the
temperature of the water.
3 Measure 5 tsp of Epsom salts. Add the
Epsom salts to the beaker and immediately record the temperature while
stirring the contents of the cup.
MATERIALS
•
•
•
•
•
•
•
graduated cylinder
hot tap water
plastic cup
thermometer
stopwatch
plastic spoon
Epsom salts
4 Continue to record the temperature
every 30 seconds for 2 minutes.
WHAT DO YOU THINK?
• What happened to the temperature
after you added the Epsom salts?
• What do you think caused this
change to occur?
Chemical reactions release or absorb energy.
COMBINATION NOTES
Use combination notes
to organize information on
how chemical reactions
absorb or release energy.
Chemical reactions involve breaking bonds in reactants and forming
new bonds in products. Breaking bonds requires energy, and forming
bonds releases energy. The energy associated with bonds is called bond
energy. What happens to this energy during a chemical reaction?
Chemists have determined the bond energy for bonds between
atoms. Breaking a bond between carbon and hydrogen requires a
certain amount of energy. This amount of energy is different from
the amount of energy needed to break a bond between carbon and
oxygen, or between hydrogen and oxygen.
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86 Unit: Chemical Interactions
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Energy is needed to break bonds in reactant molecules.
Energy is released when bonds are formed in product molecules.
By adding up the bond energies in the reactants and products, you
can determine whether energy will be released or absorbed.
If more energy is released when the products form than is
needed to break the bonds in the reactants, then energy is released
during the reaction. A reaction in which energy is released is called
an exothermic reaction.
If more energy is required to break the bonds in the reactants than
is released when the products form, then energy must be added to the
reaction. That is, the reaction absorbs energy. A reaction in which
energy is absorbed is called an endothermic reaction.
These types of energy changes can also be observed in different
physical changes such as dissolving or changing state. The state
change from a liquid to a solid, or freezing, releases energy—this is
an exothermic process. The state change from a solid to a liquid, or
melting, absorbs energy—this is an endothermic process.
Check Your Reading
The white clouds of
water vapor are formed
by the exothermic reaction between hydrogen
and oxygen.
How are exothermic and endothermic reactions different?
Exothermic reactions release energy.
2H2 + O2
Exothermic chemical reactions often produce an increase in temperature. In exothermic reactions, the bond energies of the reactants are
less than the bond energies of the products. As a result, less energy is
needed to break the bonds in the reactants than is released during the
formation of the products. This energy difference between reactants
and products is often released as heat. The release of heat causes a
change in the temperature of the reaction mixture.
Even though energy is released by exothermic reactions, some
energy must first be added to break bonds in the reactants. In exothermic reactions, the formation of bonds in the products releases more
energy. Overall, more energy is released than is added.
2H2O
The thermite reaction
releases enough heat
to weld pieces of iron
together.
Some reactions are highly exothermic. These reactions produce a
great deal of heat and significantly raise the temperature of their surroundings. One example is the reaction of powdered aluminum metal
with a type of iron oxide, a reaction known as the thermite reaction.
The equation for this reaction is
2Al + Fe2O3
Al2O3 + 2Fe
This reaction releases enough heat to melt the iron that is produced.
In fact, this reaction is used to weld iron rails together.
Check Your Reading
What is evidence for an exothermic chemical reaction?
Chapter 3: Chemical Reactions 87
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Exothermic Reactions
The products have greater bond energies
than the reactants.
Methane Combustion
reactants
H
CH4 + 2O2
O
O
O
O
C
H
H
H
bonds broken
in reactants
energy
added
H
O
O
C
O
H
H
bonds formed
in products
O
C
O
O
CO2 + 2H2O
H
energy
released
O
H
H
O
H
More energy is
released than added.
H
What information in the diagram shows that methane
combustion is exothermic?
All common combustion reactions, such as the combustion of
methane, are exothermic. To determine how energy changes in this
reaction, the bond energies in the reactants—oxygen and methane—
and in the products—carbon dioxide and water—can be added and
compared. This process is illustrated by the diagram shown above.
The difference in energy is released to the surrounding air as heat.
These jellyfish glow
because of exothermic
chemical reactions.
Some chemical reactions release excess energy as light instead
of heat. For example, glow sticks work by a chemical reaction that
releases energy as light. One of the reactants, a solution of hydrogen
peroxide, is contained in a thin glass tube within the plastic stick.
The rest of the stick is filled with a second chemical and a brightly
colored dye. When you bend the stick, the glass tube inside it breaks
and the two solutions mix. The result is a bright glow of light.
Exothermic chemical reactions also occur in
living things. Some of these reactions release
energy as heat, and others release energy as
light. Fireflies light up due to a reaction that
takes place between oxygen and a chemical
called luciferin. This type of exothermic reaction is not unique to fireflies. In fact, similar
reactions are found in several different species
of fish, squid, jellyfish, and shrimp.
Check Your Reading
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88 Unit: Chemical Interactions
In which ways might an exothermic
reaction release energy?
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The bombardier beetle, shown in the photograph on
the right, uses natural exothermic reactions to defend
itself. Although several chemical reactions are involved,
the end result is the production of a hot, toxic spray.
The most important reaction in the process is the decomposition of hydrogen peroxide into water and oxygen.
2H2O2
2H2O + O2
When the hydrogen peroxide rapidly breaks down,
the hot, toxic mixture made by the series of reactions is pressurized by
the oxygen gas from the reaction in the equation above. After enough
pressure builds up, the beetle can spray the mixture.
Endothermic reactions absorb energy.
Endothermic reactions often produce a decrease in temperature.
In endothermic reactions, the bond energies of the reactants are greater
than the bond energies of the products. As a result, more energy is
needed to break the bonds in the reactants than is released during the
formation of the products. The difference in energy is usually absorbed
from the surroundings as heat. This often causes a decrease in the
temperature of the reaction mixture.
reading tip
The prefix endo- means
“inside.”
All endothermic reactions absorb energy. However, they do not all
absorb energy as heat. One example of an endothermic reaction of this
type is the decomposition of water by electrolysis. In this case, the energy
that is absorbed is in the form of electrical energy. When the electric
current is turned off, the reaction stops. The change in energy that
occurs in this reaction is shown below.
Endothermic Reactions
The products have lower bond energies
than the reactants.
Electrolysis of Water
reactants
O
H
2H2O
H
energy
added
O
H
bonds
broken in
reactants
H
H
H
O
O
H
bonds formed
in products
2H2 + O2
H
O
O
H
H
H
H
energy
released
More energy is added
than released.
What information in the diagram shows that the
decomposition of water is endothermic?
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Probably the most important series of endothermic reactions
on Earth is photosynthesis. Many steps occur in the process, but the
overall chemical reaction is
6CO2 + 6H2O
C6H12O6 + 6O2
Unlike many other endothermic reactions, photosynthesis does
not absorb energy as heat. Instead, during photosynthesis, plants
absorb energy from sunlight to turn carbon dioxide and water into
oxygen and glucose, which is a type of sugar molecule. The energy is
stored in the glucose molecules, ready to be used when needed.
Check Your Reading
How can you determine if a reaction is endothermic?
Exothermic and endothermic reactions work
together to supply energy.
When thinking about exothermic and endothermic reactions, it is
often useful to consider energy as part of the reaction. An exothermic
reaction releases energy, so energy is on the product side of the chemical
equation. An endothermic reaction absorbs energy, so energy is on the
reactant side of the chemical equation.
Exothermic Reaction
VISUALIZATION
CLASSZONE.COM
View examples of
endothermic and
exothermic reactions.
Reactants
Products + Energy
Endothermic Reaction
Reactants + Energy
Products
As you can see in the general reactions above, exothermic and
endothermic reactions have opposite energy changes. This means that
if an exothermic chemical reaction proceeds in the opposite direction,
it becomes an endothermic reaction that absorbs energy. Similarly, if
an endothermic reaction proceeds in the opposite direction, it
becomes an exothermic reaction that releases energy.
Check Your Reading
What happens when an exothermic reaction is reversed?
A large amount of the energy we use on Earth comes from the Sun.
This energy includes energy in fossil fuels such as coal and petroleum,
as well as energy obtained from food. In all of these cases, the energy
in sunlight is stored by endothermic reactions. When the energy is
needed, it is released by exothermic reactions.
This combination of reactions forms a cycle of energy storage and
use. For example, examine the photosynthesis equation at the top of
the page. If you look at this equation in reverse—that is, if the direction
of the arrow is reversed—it is a combustion reaction, with oxygen and
glucose as the reactants, and it is exothermic.
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90 Unit: Chemical Interactions
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Plants store energy through the endothermic reactions of photosynthesis. Living things can release this energy through a series of
exothermic reactions that will be described in the next section.
Plants such as trees store
energy through photosynthesis. Cars and trucks
release this energy
through combustion.
The energy stored in plants through photosynthesis can also be
released in other ways. Consider energy from fossil fuels. Fossil fuels
include petroleum, natural gas, and coal. These substances formed
from fossilized materials, mainly plants, that had been under high
pressures and temperatures for millions of years. When these plants
were alive, they used photosynthesis to produce glucose and other
molecules from carbon dioxide and water.
The energy stored in the bonds of these molecules remains, even
though the molecules have changed over time. The burning of gasoline in a car releases this energy, enabling the car’s engine to work.
Similarly, the burning of coal in a power plant, or the burning of
natural gas in a stove, releases the energy originally stored by the
endothermic series of photosynthesis reactions.
Check Your Reading
How can endothermic and exothermic reactions work together?
KEY CONCEPTS
CRITICAL THINKING
1. What are the differences
between exothermic and
endothermic reactions?
4. Synthesize Describe the
connections between the
processes of photosynthesis
and combustion.
2. Is the combustion of methane
an exothermic or endothermic
reaction? Explain.
3. Is photosynthesis an exothermic
or endothermic reaction?
Explain.
5. Communicate Explain how
most energy used on Earth can
be traced back to the Sun.
CHALLENGE
6. Synthesize Electrolysis of
water is endothermic. What
does this indicate about the
bond energy in the reactants
and products? What happens
when this reaction is reversed?
Chapter 3: Chemical Reactions 91
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